1. Field of the Invention
This invention relates to a sonobuoy which will reduce the noise caused by placing the sonobuoy in a high shear current environment. Specifically, the sonobuoy reduces the noise by placing the hydrophone at a depth where the velocity of the current at the hydrophone is the same as the drift velocity of the sonobuoy system.
2. Description of the Related Art
In general, sonobuoys consist of four elements: a surface float, a hydrophone, a terminal weight, and a drogue. All of these parts, save the hydrophone, are used to position the hydrophone at a desired listening depth. In order to be effective, the hydrophone should remain as still as possible while at this depth. Any movement of the hydrophone could result in unwanted noise which decreases the effectiveness of the hydrophone. There are two basic motions which cause this unwanted noise: a vertical motion noise associated with vertical motion, and a horizontal motion noise associated with horizontal motion. The vertical motion is caused by surface waves interacting with the surface float, whereas the horizontal motion is caused by the varying current velocities at different water depths.
As discussed in U.S. Pat. No. 3,711,821 to Dale, some prior sonobuoys reduced the vertical motion by- attaching an elastic portion, called the compliant portion, to the cable between the hydrophone and the surface float, and attaching a terminal weight below that compliant portion. This compliant portion acted to dampen the effects of the vertical motion caused by the surface waves. Other systems utilized dampening systems such as that described in U.S. Pat. No. 5,020,032 to Dale U.S. Pat. No. 3,543,228 to Farmer. These systems reduced the vertical motion, and the associated vertical motion noise. However, the prior art failed to adequately reduce the noise caused by the horizontal motion.
The horizontal motion noise is caused by the variations in the current profile. As a whole, the sonobuoy drifts in the current at a constant drift velocity. However, depending on the depth, different parts of the sonobuoy are exposed to a variety of current velocities. This situation results in a difference between the drift velocity of the sonobuoy and the current velocity at the depth of each part. This difference in velocity is called the relative velocity for that part. Thus, the hydrophone has a relative velocity which is the difference between the current velocity at the depth of the hydrophone and the drift velocity of the sonobuoy. It is the relative velocity of the hydrophone which causes the horizontal motion noise which interferes with the effective operation of the sonobuoy system.
Prior solutions to reduce horizontal motion noise, such as those proposed in U.S. Pat. No. 3,711,821 to Dale, U.S. Pat. No. 3,992,737 to Duel, and U.S. Pat. No. 3,922,989 to McEachem, involved attaching a large drag area, called a drogue, above and near the hydrophone. By properly sizing the drogue, the relative velocity of the drogue is reduced. Since the hydrophone is near the drogue, the relative velocity of the hydrophone is also reduced, which then reduces the horizontal motion noise. The larger the drogue area, the more it reduced the relative velocity of the drogue, and the more it reduced horizontal motion noise. However, where the relative velocity is high, such as in a high current shear environment, the drogue area would have to be so large as to be impracticable. Therefore, prior art designs could only be operated at a reduced effectiveness in these high current shear environments.